Rapid proliferation of smartphones, tablets, and mobile applications has led to a tremendous increase in mobile data traffic in the last few years. For example, mobile data traffic on major mobile carriers in the United States has increased by more than 20,000% in five years. Furthermore, according to forecasts by major equipment manufacturers, this trend is likely to continue in future with a 78% compound annual growth rate. In contrast, with regulations and other constraints, the capacity of cellular networks, especially the wireless spectrum, has not increased proportionally.